Continuous Improvement in Tablet Coating and Dry Granulation

High efficiency in unit operations is a prequisite for achieving overall improvements in solid-dosage manufacturing. The authors examine recent advances in equipment design and operation in spraying, drying, and mixing to improve the tablet-coating process.

L.B. Bohle

Cost is a crucial issue for pharmaceutical companies, generic-drug manufacturers, and CMOs, which must be attentive to overall operational costs of equipment as well as upfront capital costs. High-efficiency processing in solid-dosage manufacturing is a desired goal, and tablet coaters and granulators have undergone technology improvements to achieve continuous improvement in these operations.

Improvements in solid-dosage manufacturing

To evaluate improvements in solid-dosage manufacturing, one must examine the advances in equipment design and operation in spraying, drying, and mixing to improve the overall tablet-coating process. Highlights of such advances are further described.

Tablet coating. Improvements to tablet-coating technologies are helping pharmaceutical manufacturers to achieve higher uniformity and waste reduction. These improvements are mostly due to mixing-system advances as well as pan and airflow configuration. Tablet coating consists of three main steps: spraying, drying, and mixing. Each step must be calibrated with the others to optimize the entire process.

Spraying. Spraying equipment includes the nozzle types, number and type of guns, and gun-to-gun spacing. Process parameters include spray rate, gun-to-tablet-bed distance and angle, atomization air pressure, and pattern air pressure. Spray-gun nozzles are key to efficient coating, and nozzles that create an oval spray pattern and a spray angle of 50° to 60° work best. The configuration of the spray boom will dictate the amount of spray-cone overlap. A slight overlap of the spray cones from each nozzle may be desirable for consistent coverage; however, too much overlap will lead to localized overwetting of the tablet bed. In most cases, the gun-to-bed distance will range from 6 in. to 10 in. although the actual distance depends on the force of the atomization air pressure and pattern air pressure on the coating-pan size.

Drying. Drying of the coating suspension takes place during the atomization and when spreading on the tablet surface. The temperature of the inlet air, volume flow and humidity of the inlet air, and the degree of pan perforation control drying. These parameters must be controlled in a way that prevents overwetting but does not cause spray-drying. The way the air moves through the coating pan also influences drying. Most coating pans draw air from the top of the pan and exhaust it to the side. Others place both the inlet and outlet at the pan's base. No matter where the inlet and outlet are, the goal is to handle the air in a way that minimizes turbulence and excessive heat in the spray zone. A balance must be struck between the temperature and volume of the inlet air and the spray rate.

Mixing. Mixing is a prerequisite for a homogeneous and uniform coating. Ideally, the pan's action will mix the tablets in a way that exposes each to the spray equally. Doing so depends on pan geometry and the use of baffles, spirals, lifting bars, or other components. Many common pans have a diameter that exceeds their length. This ratio can create a tablet bed too deep for effective mixing. It is not uncommon to find dead spots in the middle of these tablet beds. Such dead spots can cause overwetting and the formation of clumps or twins, which may damage the tablets.

To break up or prevent clumps and dead zones, some pans are fitted with a device called a baffle or shovel. The shovel is fixed within the coating pan, so as the pan turns, the tablets contact the shovel, steering them outward. Although the shovel insert may improve mixing, it can increase the risk of tablet breakage. An alternative approach is to use a pan whose length exceeds its diameter and that incorporates helical baffles for gentle tablet mixing. The longer pan creates a shallow tablet bed that is free of dead zones. The upper and lower helical baffles expose all the tablets to the spray equally. This configuration enables higher spray rates, which shortens the time required to coat tablets in each batch.